Field of the Invention
This invention relates to an apparatus and a method for launching plugs used in the construction of an oil or gas well.
In the construction of an oil or gas well, a hole is drilled in the ground. A casing string is then lowered into the ground and hung from the surface. The casing string is then cemented into the wellbore. The cement is pumped down through the casing string and forced out of the bottom of the casing string through a float shoe and a float collar and up the annulus formed between the external surface of the casing string and the internal wall of the wellbore. A first plug is usually launched from the top of the string ahead of the cement. A second plug is usually launched after the cement to act, amongst other things, as a barrier between the cement and the fluid being used to force the cement down the casing, and also to wipe clean the inside of the casing.
In the preparation of certain wells, a hole is drilled through the ground below a casing string. A liner of smaller diameter than the casing is then lowered through the casing and hung from the bottom end of the casing string. The liner is then cemented into the wellbore. The cement is pumped down through the liner and forced out of the bottom of the liner through a float shoe and float collar and up the annulus formed below the external surface of the liner and the internal wall of the wellbore or of the internal surface of the casing. A first plug is usually launched from the top of the liner ahead of the cement. At second plug is usually launched after the cement to act, amongst other things, as a barrier between the cement and the fluid being used to force the cement down the liner and also to wipe clean the inside of the casing.
One of the problems of existing apparatus is ensuring that plugs are launched reliably.
A bursting disk is disclosed in FIG. 27 of Applicants"" PCT Publication No. WO 96/34175. However, this is used to facilitate the separation of two plugs which may have been accidentally launched together.
The invention attempts to solve this problem.
Accordingly there is provided an apparatus for launching a plug, the apparatus comprising a canister connectable to a running string, a plug retained in. said canister, and a tube having a bursting disk, the arrangement being such that, in use, an increase in fluid pressure in said tube ruptures said bursting disk allowing an increase in fluid pressure above said plug to launch said plug.
Preferably, the apparatus further comprises a landing seat in said tube arranged below said bursting disk, the arrangement being such that, in use, when a dart lands on said landing seat, pressure is increased above said dart to rupture said bursting disk allowing an increase in fluid pressure above said plug to launch said plug.
Advantageously, the tube has an upper and a lower end, said lower end being attached to said plug and said upper end being slidably arranged with respect to a second tube.
Preferably, there is a sealing ring arranged between said tube and said second tube.
Advantageously, there is provided a further bursting disk in said tube arranged below said landing seat. The further bursting disk is designed to burst at a higher pressure than the bursting disk. The further bursting disk ruptures after the plug lands on a float collar, float shoe or other obstacle in the casing or liner and after a considerable pressure has been applied. This would then allow cement to flow through the plug and into the annulus between the casing or liner and the wellbore or casing.
Preferably, said tube has a portion which extends from said landing seat to retain said dart.
Advantageously, at least one port is provided in said canister above said plug. The port aids smooth ejection of the plug.
Preferably, said tube is substantially concentric with said plug and there is provided an annulus therebetween. The annulus should not be continuous along the length of the plug.
If so desired the apparatus may comprise a second plug which is attached to said second tube, said second tube having a second bursting disk and being slidable with respect to a third tube.
Advantageously, the apparatus further comprises a second landing seat in said second tube arranged below said second bursting disk, the arrangement being such that, in use, when a second dart lands on said second landing seat, pressure is increased above said second dart to rupture said second bursting disk allowing an increase in fluid pressure above said second plug to launch said second plug.
Preferably, a sealing ring is arranged between said second tube and said third tube.
Advantageously, there is provided a further bursting disk in said second tube arranged below said second landing seat.
Preferably, at least one port is provided in said canister above said second plug. The ports may be placed between the plugs to aid separation thereof.
Advantageously, said second tube has a portion which extends from said second landing seat to retain said second dart.
Preferably, said second tube is substantially concentric with said second plug and there is provided an annulus therebetween.
If so desired the apparatus further comprises a third plug which is attached to said third tube, said third tube having a third bursting disk and which third tube is slidable with respect to a fourth tube.
Advantageously, the apparatus further comprises a third landing seat in said third tube arranged below said third bursting disk, the arrangement being such that, in use, when a third dart lands on said third landing seat, pressure is increased above said third dart to rupture said third bursting disk allowing an increase in fluid pressure above said third plug to launch said third plug.
Preferably, a sealing ring arranged between said third tube and said fourth tube.
Advantageously, there is provided a further bursting disk in said tube arranged below said third landing seat.
Preferably, least one port is provided in said canister above said third plug.
Advantageously, said third tube has a portion which extends from said third landing seat to retain said third dart.
Preferably, said third tube is substantially concentric with said third plug and there is provided, an annulus therebetween.
Advantageously, said second tube is shear pinned to said third tube in a two plug arrangement.
Preferably, said third tube is shear pinned to said fourth tube in a three plug arrangement.
Advantageously, said canister comprises at least one retaining rib for each of said plugs.
Preferably, each of said plugs comprises fins and wherein said canister comprises at least one retaining rib for each of said fins.
There is also provided a method of launching a plug from a canister, which method comprises the steps of blocking a tube; increasing pressure in said tube above said blockage to a predetermined pressure at which a bursting disk in said tithe ruptures allowing fluid pressure to increase above said plug and launches said plug.
In a two or more plug arrangement the method comprises the further step of increasing pressure between said plug and second plug launching said plug.
In a three or more plug arrangement the method comprises the further step of increasing pressure between said second plug and third plug launching said second plug.